1. Field of the Invention
This invention relates to an improved method for synthesizing crystalline zeolites requiring a reaction mixture for crystallization thereof which contains an organic nitrogen-containing cation source. Zeolites which may be advantageously synthesized by the present improved method are characterized by an alumina to silica mole ratio of not greater than 0.083 and a constraint index within the approximate range of 1 to 12. The present improved method requires a zeolite reaction mixture composition comprising a mole ratio of hydroxide ion/silica of zero by conventional calculation procedures, a mole ratio of H+(additional)/silica of between 0 and 1.0, and a reaction mixture of pH of at least about 7.
This invention further relates to an improved crystalline zeolite product of the improved method of synthesis and to organic compound conversion in the presence of the improved zeolite as catalyst.
2. Description of the Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversions.
Certain zeolitic materials are ordered, porous crystalline zeolites having a definite crystalline structure within which there are a large number of smaller cavities which may be inter-connected by a number of still smaller channels. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline zeolites. These zeolites can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li is equal to unity. One type of cation may be exchanged either entirely or partially by another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given zeolite by suitable selection of the cation. The spaces between the tetrahedra are usually occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. A number of these zeolites require the presence of a source of organic nitrogen-containing cations in the reaction mixture used to prepare them. Those zeolites include, for example, zeolite ZSM-5 (U.S. Pat. Nos. 3,702,886 and Re 29,948), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-23 (U.S. Pat. No. 4,076,842), zeolite ZSM-35 (U.S. Pat. No. 4,016,245), zeolite ZSM-38 (U.S. Pat. No. 4,046,859), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZK-22 (U.S. Pat. No. 3,791,964), zeolite "alpha" (U.S. Pat. No. 3,375,205), zeolite "beta" (U.S. Pat. No. 3,308,069), a synthetic erionite (U.S. Pat. No. 3,699,139) and a synthetic offretite (U.S. Pat. No. 3,578,398). A method for synthesizing ZSM-5, ZSM-12, ZSM-35 and ZSM-38 is disclosed in U.S. Pat. No. 4,151,189.
Applicant knows of no prior art methods of crystalline zeolite synthesis, said synthesis requiring a source of organic nitrogen-containing cations in the reaction mixture used therein, utilizing the present improvement.